A simple form of optical network of the kind described includes an optical source (typically a laser) and a modulator at each transmitting terminal and a second optical source together with homodyne or heterodyne detecting apparatus at each receiving terminal. The optical sources associated with the transmitting terminals each provide carrier signals with different wavelengths which are modulated by an information signal and then transmitted to the cross-coupling means. This distributes the received modulated carrier signals equally between all receiving terminals. In order to receive a transmitted signal, the optical source associated with a receiving terminal is tuned appropriately.
A practical difficulty of building a network of the kind described above is that each laser associated with a transmitter must be referenced to the other to prevent cross-talk interference. Thus, if one carrier signal laser were to drift not only would the receiving terminal have to follow it, but beyond certain limits it would cause interference with other channels. Clearly it is very difficult to reference this large number of optical sources.
In accordance with the present invention, in an optical network of the kind described, the plurality of first souces of optical signals optically coupled with the transmitting terminals are grouped together.
In this arrangement the referencing of the first optical sources is much less difficult since the sources are grouped together, preferably at a central location. Also, the terminal equipment is simplified so that only a modulator is required in the transmitting terminal. A further advantage is that the grouped components can be put on an optical integrated circuit.
Where the network is to be used solely in a broadcast configuration, the transmitting terminals may be grouped together with the first sources of optical signals. In other arrangements, however, the first sources are grouped together remotely from the transmitting and receiving terminals.
In some cases, the selection means may comprise a wavelength selector such as a grating demultiplexer thus allowing direct detection to be achieved. Conveniently, this selection means is located adjacent the grouped first sources.
In another example, the selection means comprises a plurality of second sources of optical signals grouped together remotely from and optically coupled with the receiving terminals to supply tuning signals to the receiving terminals.
In one example, respective first and second sources of optical signals are coupled with each of the receiving and transmitting terminals. In this example, each of the optical sources coupled with the receiving terminals is preferably tunable in order to select the channel which is received by the corresponding receiving terminal. This is particularly useful in the case of for example a cable TV network where a particular receiving terminal may only receive certain channels.
In another example, the network further comprises switch means for optically coupling selected ones of the first optical sources with selected transmitting terminals.
In this example, where second optical sources are provided, the switch means is conveniently adapted to optically couple selected ones of the second optical sources with selected receiving terminals.
In most cases, there will be one first optical source for each transmitting terminal and, where appropriate, one second optical source for each receiving terminal. In some cases, however, there may be a fewer number of otical sources than terminals. In these cases, it is preferable if the switch means is operable on a time sharing basis.
The provision of switch means provides a more versatile arrangement than the previous example since each optical source can be of a fixed wavelength but may be fed to any of the receiving and transmitting terminals upon appropriate operation of the switch means. In the case where first and second optical sources are provided, these are conveniently arranged in pairs of transmit and receive sources separated in wavelength by the IF frequency to enable heterodyne detection to be carried out. The switch means controls not only which pair of optical sources are selected but also the transmitting and receiving terminals to which they are connected.
Since the switch means only operates on unmodulated carrier signals, if any cross-talk exists it is likely to be less troublesome than in arrangements where the information signal is being switched. If the switch means is polarisation sensitive in its performance (e.g. LiNbO.sub.3) then there is a good prospect that the polarisation can be closely controlled because the optical source and switch means are conveniently sited together. For example, the optical sources and switch means may be connected together by a short length of optical fibre which could be polarisation holding without incurring a high cost penalty.
In most practical situations, the network will include one or more transmitting/receiving stations each comprising a pair of transmitting and receiving terminals. This may be, for example, a telephone network.
In this situation, it will normally be necessary to provide three optical waveguides for each station. That is, one to conduct the carrier signal to the transmitting terminal, one to carry the modulated carrier signal to the cross-coupling means and one to carry a signal from the cross-coupling means to the receiving terminal.
Preferably, therefore, the or each station is connected with the first and second sources via a single duplex optical waveguide, the network further comprising wavelength multiplexing means for feeding multiplexed carrier and tuning signals from the first and second optical sources to the or each single optical waveguide. With this arrangement, the number of optical waveguides can be reduced to one thus simplifying the network structure and requiring the simple addition of a conventional wavelength multiplexer.
The only disadvantage of the network described above arises when a large number of terminals are involved. This is because large losses may occur.
To deal with this optical amplifiers could be used. Alternatively one of the first sources may be arranged to feed a common optical carrier signal to all the transmitting terminals, the network further comprising a plurality of auxiliary receiving terminals grouped together for receiving a modulated common carrier signal from respective transmitting terminals and for controlling modulating means to modulate another carrier signal associated with the transmitting terminal in response to the received modulated common carrier, the modulated associated carrier signal being fed to the cross-coupling means.
Referencing a bank of optical sources (such as lasers) will add a degree of complexity and cost to an optical system, especially if the bank needs referencing to other remote banks. Hence a central referencing system is attractive. If the optical sources are sited remote from the star point (i.e. the position in the network where the signals are distributed) at say a parent exchange not only are many optical waveguides needed to carry the reference signals individually to the star point but also polarisation control becomes more difficult as, for example, the performance of a L.sub.i N.sub.b O.sub.3 switch is polararisation dependent. Alternatively, a wavelength multiplex might be formed to carry the signals over a single waveguide and then each carrier separated by a demultiplexer before being fed to the switch means (if present). However with closely spaced carriers this too appears difficult.
Preferably, the network further comprises locking means for maintaining the wavelength of the optical signals from the first optical sources substantially constant. For example, a set of semiconductors lasers may be locked at frequencies offset from a stabilised and central reference such as a HeNe. The locking could be by opto-electronic phase locked loop or by injection locking lasers with a frequency shift by optical or electrical means. Alternatively it may be possible to generate a referenced set of carriers at a central point, to transmit these as a wavelength multiplex over a tree structured waveguide network to the star points and to use them to lock a set of lasers sited at the star point.
Conveniently, therefore the locking means comprises a plurality of second, remote sources of optical signals for supplying reference carrier signals with respective unique identifiers to the first sources, the network further comprising tuning control means for adjusting the output of the first sources to correspond with identified reference signals.
The identifiers could be, for example, by frequency modulation of the optical sources with a set of low frequency orthogonal signals. At the star point each first source locks on to one of the reference carriers and also bears the identifier imprint. This can be read and used to indicate to the local tuning control means whether returning needs to take place. As returning can take place before the switch means operates (when present), spurious laser signals can be prevented from appearing in the signal network. Additional benefits from this type of scheme are
(1) the first sources can be tuned to any of the incoming refrence carriers and this gives an extra degree of flexibility in the network operation; PA1 (2) as the identifiers must form an othogonal set, it may be possible to modulate these to provide additional information channels, for example, for control signals.
In theory, the optical waveguides used to couple the terminals and sources may comprise multimode optical fibres. Preferably, however, monomode optical fibres are used in view of their high bandwidth potential and low power loss.
Furthermore, although in theory incoherent light could provide the optical signals it is preferable for these to be at least temporally coherent and conveniently the sources of optical signals comprise lasers.
In this specification the term optical is intended to refer to that part of the electro-magnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultra-violet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
The cross-coupling means may be arranged to cause modulated carrier signals from each transmitting terminal to be fed to each receiving terminal. Other arrangements are possible and indeed the cross-coupling means may be connected additionally to further cross-coupling means of other networks.